1. Field of the Invention
The present invention relates to well drilling and servicing fluids utilized in producing formations and the removal of filter cake deposited by the fluids in the formations.
2. Description of the Prior Art
The use of special fluids for drilling or servicing hydrocarbon producing formations penetrated by well bores is well known. When well bores are drilled into producing formations, drilling fluids are utilized which will minimize damage to the permeability of the formations and their ability to produce hydrocarbons. Servicing fluids are utilized when completion operations are conducted in producing formations and when conducting work-over operations in the formations. The drilling and servicing fluids deposit a layer of particles known as “filter cake” on the walls of the well bores within the producing formations. The filter cake prevents the drilling and servicing fluids from being lost into the formations and prevents solids from entering the porosities of the producing formations. The filter cake reduces operational costs by precluding the loss of fluids into the formation and preventing solids from entering the porosities of the formation. Following completion and prior to initiating production, the filter cake must be removed. Despite advances in the state of the art, filter cake removal remains a problem in the industry.
Drilling and servicing fluids usually comprise fluid loss control materials, a polymer solid suspending agent, and an acid soluble particulate solid bridging agent. One common bridging agent for bridging over formation pores is calcium carbonate. The filter cake formed by the drilling or servicing fluid includes the bridging agent, the polymeric suspending agent, and the fluid loss control material.
Heretofore the filter cake has been removed by contacting the filter cake with a strongly acidic solution for a period of time sufficient to dissolve the bridging particles and decompose the polymer. Despite current anti-corrosion steps, the strongly acidic solution often corrodes metallic surfaces and completion equipment such as sand control screens causing early failure of such equipment. The acidic solution may also be incompatible with the producing formation and cause damage thereto.
The specific gravity of calcium carbonate creates another problem for the operator. Drill solids and calcium carbonate have approximately the same specific gravity. As a result, conventional rig separation equipment, which relies in part on differences in specific gravity, is unable to remove the drill solids without also removing the bridging particles.
Thus, there are continuing needs for improved drilling and servicing fluids and methods of removing filter cake deposited by the fluids from producing formations.